The mechanisms by which nerves and muscles develop and are maintained as partners are unknown. The purposes of this research are to study how an important protein molecule localized at the nerve-muscle junction, acetylcholinesterase (AChE), is mobilized during development, and secondarily, how regulation of its activity and localization is defective in inherited muscular dystrophy of the chicken. Experiments in progress use electron microscopy to examine the ultrastructural localization of AChE in muscle and nerve cultures, chick embryo muscle, and denervated and dystrophic chicken muscle. Special emphasis is placed upon muscles recovering from treatment with diisopropylfluorophosphate, an inhibitor of AChE, to study the mobilization of newly synthesized enzyme and the pathway by which it is released from muscle fibers. This year we demonstrated that electrical stimulation reversibly reduced the AChE activity of muscle cultures and inhibited the increase of AChE activity that follows denervation of chicken muscle. The data suggest that contraction itself regulates AChE levels. Our next step is to investigate the role of excitation-contraction coupling in the regulation of AChE in muscle and examine the effect of stimulation on dystrophic chicken muscles. In order to find out whether the defect in AChE regulation of dystrophic muscle was a property of nerve or muscle, limb buds were transplanted between 3 1/2 day old normal and dystrophic embryos, and their muscles examined after hatching. The results showed that AChE activity of the limbs was a property of their genotype and not of the nerves of the host and that, at least after 3 1/2 days of incubation, nerves of dystrophic chickens do not determine the lack of regulation of AChE in the muscles. The results support the view that dystrophic chicken muscle fibers themselves are defective in control of AChE.